The while command evaluates test as an expression
(in the same way that expr evaluates its argument).
The value of the expression must a proper boolean
value; if it is a true value
then body is executed by passing it to the Tcl interpreter.
Once body has been executed then test is evaluated
again, and the process repeats until eventually test
evaluates to a false boolean value. Continue
commands may be executed inside body to terminate the current
iteration of the loop, and break
commands may be executed inside body to cause immediate
termination of the while command. The while command
always returns an empty string.

Note: test should almost always be enclosed in braces. If not,
variable substitutions will be made before the while
command starts executing, which means that variable changes
made by the loop body will not be considered in the expression.
This is likely to result in an infinite loop. If test is
enclosed in braces, variable substitutions are delayed until the
expression is evaluated (before
each loop iteration), so changes in the variables will be visible.
For an example, try the following script with and without the braces
around $x<10: